1. Field of the Invention
The invention relates to the field of fastening systems and, in particular, to preload attenuation devices that either reduce a preload on a assembled joint or applies a preload between joined structures and upon command, reduces the preload.
2. Description of Related Art
On satellites launched from ground based boosters, it is necessary to insure that the satellite and it's components will survive the large vibration and acoustic G forces produced during the first few minutes of flight. This requires that the satellite and its components be attached with highly preloaded fasteners. Unfortunately, this configuration can cause problems. Upon reaching orbit the satellite must be separated from it's booster. In addition, components of the satellite, such booms and antennas must thereafter be released. The separation of the booster rocket and deployment of the components commonly accomplished by the use of explosively actuated release systems. While these explosive devices are highly reliable, they can cause significant shock loads to be introduced throughout the satellite. Thus what is necessary is a fastening system that can produce a high preload and then, upon command or passively relax the preloads prior to actuation of these shock-inducing systems.
Shape Memory Alloys (SMA) offers a solution to this problem. There are numerous alloys having shape memory characteristics such as Ti-Ni, Au-Cd, In-Zn, Ti-Ni-Cu, Cu-Zn-Al and Cu-al-Ni and many are commercially available. There are basically two types of SMA actuation modes or memory configurations.
1. The One Way Shape Memory effect is defined as: Setting a desired shape through a heat treat process then mechanically deforming the SMA element to another specified configuration, while maintained in a cold or martensitic finished condition and not exceeding certain mechanically induced strain constraints. When the SMA element is then heated to and through its phase transformation temperature, austenitic start through finish, it will then return to its previous memory shape that was set during the heat treatment process. When cooled to it's martensitic temperature the SMA element will not change to any other intermediate shape and will remain in the original heat-treated configuration.
2. The Two Way Shape Memory effect is defined as: Setting an intermediate shape configuration through a heat treat process then mechanically deforming and repetitively training the SMA element to another specified configuration and by doing so, establishing a desired cold memory martensitic finished shape memory with a much different austenitic finish hot memory shape. With this two way memory established, the SMA element will then be able to transition from one memory shape to the other by transitioning from martensitic through austenitic with no requirement for an external mechanical strain to make a shape change. Upon cooling from austenitic too martensitic the process can then be reversed and the SMA element reverts back to it's cold martensitic shape configuration.
3. With this two way memory established, there is an added benefit to use the two-way memory effect while employing a Stress Induced martensite phenomena. This process will gives SMA element a greater displacement travel or shape change recovery capability through the martensitic to the austenitic shape change. This process would be classified as a cocking procedure, where as an additional bias strain is induced in the direction of the hot to cold shape change during the austenitic to martensitic cycle. This added travel or work will not repeat in subsequent two way memory cycles unless the added bias strain is employed again at the time of cooling.
There have been numerous fastener designs using shape memory alloys. For example, U.S. Pat. No. 4,450,616 "Method Of Ensuring The Tightness Of a Bolt And A Nut" by K. Morita discloses a method of preloading a joint using a simple washer made of shape memory metal. The washer is formed in the austenitic phase using a press. It is then cooled to a temperature lower than the temperature for starting an austenitic transformation. Thereafter the washer is installed and allowed to return to the higher temperature causing it to recover and expand preloading the joint. However, this washer is not suitable for applications where it is desirable to have the preload reduced after a specific amount of time. A similar washer is disclosed in U.S. Pat. No. 5,791,847 "Washer And Method Of Using The Washer" by J. M. Keto-Tokoi
U.S. Pat. No. 4,897,006 "Device for exerting A Pressure Developing Stages On A Part" by M. Blin proposes that washers with varying critical transformation temperatures be stacked together to provide different preloads depending upon the temperature. The primary use of this invention contemplated by the inventor is in the fabrication of composite structures where the stack of washers can be used to control the pressure load on a part being formed within the mold. U.S. Pat. No. 5,094,551 "Preload Control Apparatus For Bearings With Shape Memory Alloy Springs" by K. K. Katsuji, et al. also discloses the use of springs of vary transformation temperatures to vary the preload on a bearing as a function of the bearing temperature.
U.S. Pat. No. 5,248,233 "No-Shock Separation Device" by R. G. Webster is more to the point of this invention in that is discloses a fastener assembly for relieving preloads. In detail, the patent to R. G. Webster discloses a fastener assembly that includes a segmented nut kept in engagement with the threads of a bolt by a spring-biased ring thereabout. A shape memory metal column (washer) is positioned under the nut that includes heating elements used to heat the column to above its transition temperature, such that it contracts and reduces the preload on the bolt. The contraction also releases the retainer allowing the segmented bolt to separate from the nut releasing the bolt. This fastener has a disadvantage in that the bolt is freed as the preload is reduced. There are numerous applications where it is desired to only reduce the preload, but still maintain the joint. In addition, there are applications where it is desirable to only maintain the preload for a short period and therefore reducing the preload without the application of electrical current. Finally, the separation point can not be precisely timed because of the variables in heating rates produced by the heating elements.
Thus, it is a primary object of the invention to provide a washer for preloading structures and for attenuating the preload.
It is another primary object of the invention to provide a washer for preloading structures and for attenuating the preload that is simple to manufacture.
It is a further object of the invention to provide a washer for preloading structures and for attenuating the preload that includes redundant heating elements to affect the change in preload.
It is a further object of the invention to provide a washer for preloading structures and for attenuating the preload that includes the use of stress induced martensite for cocking or reset so as to generate a greater recovery displacement or travel upon heating.